1. Technical Field
The embodiments herein generally relate to mobile television (TV) technologies, and, more particularly, to mobile TV differentially encoded orthogonal-frequency-division-multiplexing (OFDM) systems.
2. Description of the Related Art
FIG. 1 summarizes the differential modulation procedure that is described in a differentially encoded OFDM system such as terrestrial digital multimedia broadcasting (T-DMB). The phase reference symbol is perfectly known at the receiver after the transmission mode is detected. In other words, the Quadrature phase-shift keying (QPSK) symbols in every OFDM carrier are known at the receiver. The phase reference symbol belongs to a rotated quadrature phase-shift keying (QPSK) constellation as shown in FIG. 1 for Mode I. The data to be transmitted belongs to a normal QPSK constellation and is denoted by Xi. The data that is transmitted over the OFDM carriers in each OFDM symbol is obtained according to the following equation (assuming Mode I):Di=Di−1×Xi where D1 is the phase reference symbol. The frequency-domain representation of the received symbols, Ri, at the receiver is given by Ri=Di×Hi, i=2, . . . ,76, where Hi is the channel seen by the ith OFDM symbol.
At the receiver, the soft decisions that are passed to the Viterbi decoder are obtained as follows:Ri×R*i−1=|Di−1|2×Xi×Hi×H*i−1.
In this context, “soft decisions” refers to a complex number that represents the location of the received QPSK symbol with respect to the transmitted constellation. A hard decision could simply be obtained by choosing the QPSK constellation point that is closest to the received soft decision.
The quantity |Di−1|2 does not affect the decision because it is pure real. These soft decisions would lead to a correct decision on what has been transmitted if and only if the channel has not changed from one symbol to the next. In other words, if Hi≡Hi−1, then the soft decisions will reduce to:Ri×R*i−1=|Di−1|2×Xi×|Hi|2,and effectively, Xi is fed to the Viterbi decoder. The problem now is that this assumption is not accurate. Accordingly, there remains a need for a new coherent detection technique for differentially encoded OFDM systems.